Curable resins are suitable for a variety of applications. For example, curable resins have been used as coatings, sealants and adhesives, and in producing molded articles. The manufacturing facilities that utilize conventional curable resins in their operations are typically expensive to maintain due to the nature of these resins. Conventional curable resins are typically solvent-based and contain volatile organic components (VOCs). VOCs favorably affect the viscosity of curable resins, and often VOCs are added so as to make the curable resin composition sprayable. This is a particularly desired quality for coating applications (e.g. furniture, automobile, ship, aircraft and other transportation vehicle coating applications). However, during the application and cure of these conventional resin compositions, substantial amounts of VOCs are released into the atmosphere. This is a serious problem because VOCs are toxic, flammable, explosive, smog-producing and noxious. As a result, elaborate and expensive fire and explosion-prevention measures, worker protection measures and pollution control equipment are required during the use of such conventional resins.
A number of resin manufacturers are currently developing low/no VOC curable resin formulations including, high solids, blocked isocyanate polyurethanes and aqueous-based polyurethane dispersions. These emerging resins possess certain advantageous attributes, but not without specific drawbacks. High solids formulations use low VOCs and provide equivalent performance to low solids urethanes, but cannot be applied using spraying techniques. Blocked isocyanate polyurethanes also use low VOCs and have low toxicity levels and good properties. These materials however, must be cured at elevated temperatures around 150 to 200° C., which is not practical in most applications, for example in coating automobiles and aircraft. Aqueous based polyurethanes use no VOCs, are nontoxic and easy to apply via spraying. To date, however, these polyurethanes have not provided the properties required to meet military specifications, such as adhesion and moisture-resistance, desired for high performance applications.
Polyesters have emerged as promising candidates for inclusion as components in advanced aircraft coatings. Polyesters are polymers formed by the reaction of aliphatic or aromatic polyol monomers with aromatic or aliphatic diacids in the presence of catalyst, usually, metallic acetates, such as calcium acetate. Polyesters can provide a combination of useful properties including: clarity, transparency, absence of color, flexibility, excellent adhesion to most substrates, abrasion-resistance, water-resistance, fuel and oil-resistance, UV-resistance, weather-resistance, variable hardness (from elastomer to tough plastic), moderate cost, low temperature flexibility and availability as variable molecular weight, ester-endcapped, difunctional prepolymers.
Polyester polymers cannot be used as curable resins themselves, however, since their functionality is not conducive to the formation of hard, cross-linked structures. To be used advantageously as advanced coatings, polyester polymers must first be functionalized with reactive groups, such as hydroxyl groups, and then cured by chemical reaction with other comonomers, such as polyurethane diisocyanate monomers, into hard, tough polymer resins that can be used as protective coatings. Hydroxy-functional, polyester-based reactive prepolymers are frequently selected for use as components in polyurethane coatings over polyether and other type, polymers since they typically impart superior solvent-resistance, mechanical toughness and clarity to the coating and do not yellow upon exposure to intense UV light.
The major problems associated with the use of polyester-derived polyurethanes have involved the requirement for addition of substantial quantities of flammable, toxic solvents to polyurethane coating formulations to lower viscosity sufficiently for spray-painting operations. Another problem involved the use of toxic, irritating and volatile diisocyanate monomer components as major components in the polyurethane coating. The vapors that evolve from polyurethane operations may cause serious injury to workers breathing the fumes if they do not wear protective equipment.
A number of polyester-polyurethane coating manufacturers have been actively pursuing the development of low VOC and no VOC polyurethane curable resin formulations. To date however, none of these resins have exhibited the required processability or properties for use in e.g., automotive and aircraft coatings.
For example, damaged areas on U.S. Air Force aircraft are currently repaired on-site using aerosol sprayed coating compositions containing single component polyurethane enamel. Generally, these compositions are sprayed on the damaged portion of the aircraft and allowed to dry.
One disadvantage associated with such methods and compositions is that substantial amounts of volatile organic solvents (VOCs) are released into the atmosphere during application. In addition, such “touch-up” methods and compositions result in coatings having significantly inferior properties as compared to the original coating. Namely, the portions of the aircraft that are repaired using these one component polyurethane enamels have inferior abrasion-resistance, UV stability and fuel/oil resistance properties as compared to the original two-component polyurethane (PUR) coating of the aircraft. This is because unlike the two-part PURs, the one component coating formulation does not cure in-situ to form a crosslinked urethane structure in place, but, rather, simply deposits as a film upon evaporation of solvent.
Two-component aliphatic polyurethane compositions have been developed which provide satisfactory performance as aircraft coatings after drying and cure. However, to date, these coating compositions require the use of large quantities of flammable, toxic and environmentally hazardous organic solvents/diluents to produce low viscosity sprayable coatings. In addition, these polyurethane coating compositions utilize pure aliphatic diisocyanate as one of the reactive components. During coating and cure operations the pure aliphatic diisocyanate component liberates toxic vapors that may harm the workers and the environment.
A number of aircraft coating manufacturers are currently developing low/no VOC coating formulations including, high solids, blocked isocyanate polyurethanes and aqueous-based polyurethane dispersions. However, to date, these new materials have either been too high in viscosity for spraying, require excessive temperatures for cure (150 to 200° C.) or simply did not provide the properties required to meet military specifications. For example, while the high solids formulations use low VOCs and provide performance equivalent to that of low solids urethanes, these formulations cannot be applied using spraying techniques. Blocked isocyanate polyurethanes also use low VOCs, have low toxicity levels and good properties. However, these materials must be cured at elevated temperatures (150 to 200° C.), which are not practical when coating aircraft, since must be coated at ambient temperature. Aqueous based polyurethanes use no VOCs, are nontoxic and easy to apply via spraying. However, to date, these compositions do not provide the properties required to meet military specifications, such as adhesion and moisture-resistance.
Furthermore, current coating repair processes do not take into account the need to replace the underlying corrosion-resistant conversion coating and primer coating layers that were originally applied to the aircraft aluminum surface skin prior to application of the PUR topcoat. During surface preparation for re-coating, most damaged coatings are currently sanded down to bare metal, thereby removing any primer and conversion coating that might have remained and exposing the metal to the threat of corrosion, even after application of topcoat.
Accordingly, what is needed are improved curable resin compositions that contain no or substantially no VOC's and methods of using these compositions whereby no VOC's are released. It is desired that these curable resin compositions are convenient to apply, and, preferably are sprayable. It is further required that these coating compositions air-dry at room temperature with or without application of a UV-source.
Specifically, in the case where the curable resin compositions are used as coatings to repair aircraft, it is desired that these coating compositions and methods provide equivalent or better performance than conventional two-component polyurethane (PUR) coatings that are used on aircraft. Namely, it is desired that these coating compositions provide excellent adhesion to aircraft surfaces after drying, protect against corrosion, UV rain erosion, small particle impact and weathering, and resist the effects of operational fluids (fuel, lubrication, hydraulic and deicing) and moisture. It would further be advantageous to provide coating compositions that may be easily removed by stripping if required.